U.S. patent number 4,249,925 [Application Number 06/035,843] was granted by the patent office on 1981-02-10 for method of manufacturing an optical fiber.
This patent grant is currently assigned to Fujitsu Limited. Invention is credited to Masao Kawashima, Bun Kikuchi, Hisanao Okada.
United States Patent |
4,249,925 |
Kawashima , et al. |
February 10, 1981 |
Method of manufacturing an optical fiber
Abstract
A method of manufacturing an optical fiber having no defect in
the refraction index profile comprising manufacturing a bare fiber
by heating and extending a core starting material and forming a
cladding layer on said core to produce a double-layer of the core
and cladding with different refraction indices. Fine granules of
oxide for the glass cladding can be formed by vapor deposition on
the bare fiber, and these can be fused to form the cladding by
heating. Another method for forming the cladding layer is to pass
the bare fiber for the core through a fine hole provided at the
bottom of a crucible in which the glass for the cladding is
contained in liquid form.
Inventors: |
Kawashima; Masao (Yokohama,
JP), Okada; Hisanao (Oyama, JP), Kikuchi;
Bun (Oyama, JP) |
Assignee: |
Fujitsu Limited (Tokyo,
JP)
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Family
ID: |
26397279 |
Appl.
No.: |
06/035,843 |
Filed: |
May 4, 1979 |
Foreign Application Priority Data
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May 12, 1978 [JP] |
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53-56334 |
May 12, 1978 [JP] |
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53-56335 |
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Current U.S.
Class: |
65/430;
65/DIG.16; 65/60.53; 65/60.8; 65/392; 65/413 |
Current CPC
Class: |
C03B
37/027 (20130101); C03C 25/1061 (20180101); C03B
37/02718 (20130101); Y10S 65/16 (20130101) |
Current International
Class: |
C03B
37/027 (20060101); C03B 37/02 (20060101); C03C
25/10 (20060101); C03C 025/02 () |
Field of
Search: |
;65/2,3A,3C,13,18,6D |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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197811 |
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Nov 1978 |
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DE |
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1134466 |
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Nov 1968 |
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GB |
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Primary Examiner: Lindsay, Jr.; Robert L.
Attorney, Agent or Firm: Staas & Halsey
Claims
What is claimed is:
1. A method of manufacturing an optical fiber comprising a core and
at least one glass cladding on said core, said method comprising
the steps of:
manufacturing said core to comprise a bare fiber by heating and
extending a starting material for said core,
coating a starting material for said at least one glass cladding in
the form of fine granules of oxide onto said core, and
forming said at least one glass cladding by heating said fine
granule oxide coating.
2. The method of claim 1, said forming of said core portion
occurring at a higher temperature than that of said at least one
cladding.
3. The method of claim 1, said heating and extending of said
starting material for said core comprising selective use of an
electric furnace, a high frequency induction furnace, a laser and a
gas burner.
4. The method of claim 1, said starting materials for at least one
of said core and cladding comprising fused silica glass.
5. The method of claim 1, said starting materials of said core and
cladding each comprising a fusible oxide type glass, and at least
one of said starting materials comprising at least one oxide
selected from the group consisting of the oxides of germanium,
phosphorus, tin, niobium, zirconium, lanthanum, fluorine, boron,
arsenic, magnesium, calcium, titanium, gallium, aluminum, antimony,
tellurium, sodium, lithium, potassium and lead in amount exceeding
0.1% wt.
6. The method of claim 1, said starting material for said cladding
comprising a small refraction index, a small thermal expansion
coefficient and a low viscosity, at the process temperature for
fusing said cladding layer, as compared with said core, and
excellent affinity with said core.
7. The method of claim 1, said cladding layer comprising SiO.sub.2
and B.sub.2 O.sub.3, and said heating of said fine granule oxide
comprising a maximum temperature between 1350.degree. and
1400.degree. C.
8. The method of claim 1, said coating of said starting material
for said cladding comprising forming said fine granules of oxide by
selectively employing a spray method, a brash coating method, a
smoke method, a vapor bathing method, a plating method with use of
an electrolyte containing said fine granules, an RF sputtering
method, and an organic resin solvent method comprising said
starting materials for said cladding layer suspended in said
solvent.
9. The method of claim 1 comprising coating said optical fiber with
another cladding layer comprising a polymer for preventing
deterioration of the mechanical strength of said optical fiber.
10. The method of claim 1 comprising heating and extending said
core portion during said forming of said at least one cladding.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing an
optical fiber to be used in the field of optical communication
systems, and particularly to a method of producing an optical fiber
having no defect in the profile of its refraction index.
2. Description of the Prior Art
A known method of manufacturing optical fiber is often employed
wherein the core and a cladding layer are first integrated into a
single unit and a fiber is then obtained by heating and expanding
this integrated unit. This method has the disadvantage that the
boundary between the core and the cladding is not clearly defined,
with a resulting loss in total reflection, and this method also has
the disadvantage that defects in the refraction index profile
result in the area near the center of the core.
Degradation of the total reflection efficiency directly affects the
attenuation characteristic of the optical fiber, and particularly
any defect in the refraction index at the area near the center of
the core portion in the single mode fiber affects significantly the
optical transmission.
Requirements which should generally be provided in an optical fiber
of this kind are:
1. The optical transmissivity of the core glass which is used as
the optical transmission line should be quite high.
2. In order to ensure highly efficient total reflection at the
boundary of the core glass and the cladding glass the boundary must
be clearly defined.
3. The refraction indices of the core and cladding glass should
satisfy certain conditions: ##EQU1## where: n1: Refraction index of
core glass
n2: Refraction index of clad glass
a: Radius of core glass
.gamma.: Optical wavelength
Generally, the following methods are well known for manufacturing
optical fibers of the core glass and cladding glass combination
type.
1. Double-crucible method:
The core glass material and cladding glass material are melted
respectively in inner and outer crucibles forming a
double-chambered crucible, and these materials are simultaneously
extracted from a fine hole at the bottom of this double-crucible
and formed into the fiber.
2. Rod-in tube method:
The core glass material is first formed into a rod and then
inserted into a tube of the cladding glass material, and then they
are both heated and fused in the form of an integrated solid rod.
Thereafter, the rod is extended in the form of a wire, so that a
fiber is obtained.
3. CVD Method (Chemical Vapor Deposition):
A starting material in gas form is thermally oxidized in a fused
silica reaction tube which is heated and rotated by means of a
lathe. The oxidized material is deposited on the inner surface of
the fused silica tube. The deposit is then formed into the glass
material. Thereby the thin film layers of the cladding glass and
core glass can be formed repeatedly. Then, these layers are formed
into a preform in the form of an integrated rod. This preform is
expanded into the form of a wire by using a heat source to form the
fiber.
However, in any of the above-mentioned known methods, the fiber is
obtained by first forming the core and the cladding portion in an
integrated body which is then heated and extended into the form of
a wire. Therefore, such methods have the disadvantage that the
boundary between the core and the cladding is not distinct so that
the total reflection coefficient is degraded.
In addition, in the process (called the collapse process) using a
CVD method where the deposited glass tube is collapsed into a solid
rod, the glass tube is heated to a temperature as high as
2000.degree. C. Thereby, both SiO.sub.2 and the additive for
controlling the refraction index (a dopant comprising GeO.sub.2,
P.sub.2 O.sub.5 etc.) are vaporized. Moreover, this method also has
another disadvantage, that the dopant (for example, GeO.sub.2) is
more easily vaporized as compared with SiO.sub.2, so that the
concentration of the additive (GeO.sub.2 in this case) for raising
the refraction index is reduced at a very thin layer on the glass
surface which becomes the core portion and results in a defect in
the refraction index profile in the area near the center of the
core portion, and simultaneously the distribution of the refraction
index is deformed. Such condition is shown in FIG. 1 where the
horizontal axis represents the radial direction in the fiber, with
the core portion 1 and the clad portion 2. The vertical axis
represents the refraction index. This figure graphically shows the
refraction index and a defect A of the distribution of the
refraction index.
Particularly, the core diameter of a single mode fiber is as small
as several microns and therefore generation of any defects in the
refraction index profile at an area near to the center of the core
portion has a distinctively bad influence on the characteristic of
optical transmission.
Therefore, currently desired is a method of manufacturing a fiber
wherein the boundary of the core and clad portions is defined
clearly, the total reflection coefficient is excellent and
simultaneously no defect in the refraction index profile is
generated in the area near the center of the core.
SUMMARY OF THE INVENTION
The present invention is aimed at eliminating the above-mentioned
disadvantages, and offers a method of manufacturing an optical
fiber of high quality in which the boundary between the core and
clad portions is distinctively defined and defects in the
refraction index profile are also eliminated.
In addition, the core material must provide minimal optical loss
and it should also have minimal optical absorption and scattering
loss characteristics. On the contrary, there is no need of paying
so much attention to the cladding material as compared with the
process for forming the core portion since such material does not
have such a bad influence on the optical transmission
characteristic.
It is another object of the present invention to offer a method of
manufacturing an optical fiber which is very effective from the
various points of view discussed.
According to an embodiment of the present invention, an optical
fiber having no defect in the refraction index profile can be
obtained. Double layer optical fibers comprising a bare fiber as
the core portion can be manufactured by heating and expanding a
starting material for the core. The core and the cladding can have
different values of refraction index. The cladding layer can be
formed on the core by vapor depositing a fine granular oxide on the
bare fiber of the core which is then heated to form the glass
cladding layer. Also such a fine granular oxide may be coated
directly onto the bare fiber surface of the core and thereby the
glass cladding layer can be formed continuously.
According to another embodiment of the present invention, the
boundary between the core and the cladding is defined clearly by
forming the glass cladding layer by melting the glass material for
the cladding layer in a crucible with a fine hole at the bottom and
then the bare fiber of the core is passed through the fine hole.
Thusly, an optical fiber having an excellent total reflection
coefficient can be manufactured.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the distribution of the refraction index of an optical
fiber with a profile defect in an optical fiber manufactured by a
housing optical fiber manufacturing method;
FIG. 2 shows a preferred embodiment of the present invention;
FIG. 3 shows another embodiment of the present invention; and
FIG. 4 shows still another embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be explained in reference to the
drawings.
FIG. 2 shows an embodiment of a device for the present invention,
namely: the starting material for the core 21; the device 22 for
feeding the starting material for the core; the high frequency
induction furnace 23; the ring type oxy/hydrogen burner 24; the
carrier gas 25; the gas phase generation system 26; the electric
furnace 27; the polymer (primary coat) coating device 28; the
electric furnace 29; the fiber extracting device 30 by which the
fiber is extracted and wound thereafter on the drum 40.
As the starting material of the core, high purity fused silica
glass which can be obtained on the market is used. The reason why a
high purity fused silica glass material is used is that many kinds
of doping materials, and excessive amounts of other doping
materials, cause an increase of optical transmission loss
(absorption loss) and easily result in defective distribution of
the refraction index as a result of vaporization and thermal
diffusion of a dopant and indistinct definition of the boundary
between the core and the cladding. Thus, a high purity fused silica
glass material is convenient since it does not contain any such
dopants.
Hereunder, the explanation will be developed sequentially in
accordance with the manufacturing process of the present
invention.
The outside of a high purity fused silica glass rod is mechanically
polished (centerless polish) and its surface is subjected to
washing with a mixed acid including hydrofluoric acid. Then, the
surface is cleaned and smoothed by a fire polish treatment and
further washed with hydrofluoric acid. Thereby a high purity fused
silica glass rod core starting material 21 with outer diameter of
10 mm and length of 500 mm can be manufactured. Consequently, it is
formed into a fiber core having an outer diameter of 100 microns by
using the high frequency induction furnace 23.
In this embodiment the high frequency induction furnace 23 is used,
but it is obvious that the current purpose can be attained by using
any heat source that is capable of heating the glass material so
that the viscocity is low enough for extending the material, for
example, an electric furnace, laser, gas burner, etc.
As for the surface treatment of the outside of the fused silica
glass rod which is used as the core material, processes other than
the above treatment may be added if necessary, and the process also
may be modified in ways obvious to one skilled in the art.
Furthermore, in this embodiment pure fused silica has been used for
the core material as the optimum example, but if required in order
to control the refraction index, thermal expansion coefficient and
softening point, a fused silica glass material having a soluble
oxide of at least one oxide of germanium, phosphorus, tin, niobium,
zirconium, lanthanum, fluorine, boron, arsenic, magnesium, calcium,
titanium, gallium, aluminum, antimony, tellurium, sodium, lithium,
potassium and lead may be used as an additive with a content
exceeding 0.1% by weight. A multi-element glass material comprising
a combination with at least one of the above oxides may also be
used.
Then, fine granules of boron-doped silica having thickness of about
50 microns are formed at the outside surface of the fiber core to
comprise an outer diameter of 100 microns by fire hydrolysis and
then the coated fiber is heated and sintered in the electric
furnace 27 to form the cladding glass layer integrated with the
core portion.
The integrated fiber may have a core diameter of 85 microns and a
cladding thickness of about 25 microns.
Cladding by means of fire hydrolysis is formed with the method
explained below. Dry oxygen which works as the carrier 25 is
inflowed to the bubbler which contains silicon tetrachloride
(SiCl.sub.4) at 30.degree. C., and to the bubbler which contains
boron tribromide (BBr.sub.3) at 30.degree. C., and then the vapors
of SiCl.sub.4 and BBr.sub.3 carried by this oxygen are mixed and
the amount of said oxygen is adjusted so that the amount of
BBr.sub.3 becomes 7.1 wt%.
Thereafter, the mixed vapor of SiCl.sub.4 and BBr.sub.3 carried by
the oxygen passes through the oxyhydrogen flame burner and is
subject to hydrolysis therein. Thereby a fine granule oxide
comprising SiO.sub.2 in about 86 wt% and B.sub.2 O.sub.3 in about
14 wt% is deposited on the surface of the core. This oxide on the
fiber is supplied to the electric furnace 27 provided in the lower
side and heated up to about 1350.degree. C. and sintered to the
core glass. Thus the glass cladding layer is formed.
The method of forming the glass cladding layer by means of fire
hydrolysis as described above is an example of one method of vapor
deposition, and it is also possible to use other methods of vapor
deposition, for example, to use a heat source such as an electric
furnace or a plasma torch.
Moreover, in the above-mentioned method, the process for depositing
the oxide and for sintering the glass are explained separately. If
desired, however, it is possible to continuously coat while
directly forming the cladding glass on the core surface by means of
a heat source utilizing such a method of vapor deposition.
Also employed as another method of forming the cladding layer is
that the glass material for the cladding is fused in a single
cavity or multi-cavity container and then the core fiber is applied
to the fused glass to form the cladding layer.
Moreover, such a cladding layer can be realized by various other
methods, namely:
A method where the fine glass granules for the cladding are
suspended in a solvent which is coated on the core by a spray
method or a brash coating method and then the cladding layer is
formed after it is heated and fused;
A method where the fine glass granules for the cladding are coated
on the core with a smoke method (or a vapor bathing method) and
then the cladding is formed after it is heated and fused;
A method where the fine glass granules for the cladding are
suspended in an electrolyte for being coated on the core and the
cladding layer is formed after heating and fusing (a plating
method);
A method where the pieces of glass to be applied as the cladding
material are bonded onto the core with an RF sputtering method
which is also used for the sputtering target and then the cladding
layer is formed after it is heated and fused; and
A method where a mixture of glass forming and coating material is
suspended in an organic resin solvent and deposited and then the
cladding layer is formed after it is heated and fused.
The composition of the glass material for cladding is related to
the composition of the core material, but as is already apparent,
it is also possible, if required, to change the amount of content
and/or composition of an additive as compared with the composition
of glass material for the core, in order to adjust the refraction
index and if necessary the thermal expansion coefficient and
softening point.
After formation of the cladding, a polymer, for example, a fluorine
family resin (KYNAR) or a silicon resin may be coated if required
in order to prevent generation of minute flaws, and thus to
maintain or prevent deterioration of mechanical strength.
FIG. 3 shows another embodiment for realizing the present
invention, namely: the starting material 31 for the core; the
device 32 for feeding the starting material for the core; the
electric furnace 33; the glass material 34 for the cladding; the
high frequency induction furnace 35; the glass fusing pot 36 for
the cladding; the polymer (primary coat) coating device 37; the
electric furnace 38; the fiber extraction device 39; and the
take-up device 40.
The manufacturing processes corresponding to these FIGS. 2 and 3
will now be sequentially explained.
The outside of a high purity glass rod is polished mechanically and
then its surface is subjected to a washing treatment with a mixed
acid including hydrofluoric acid.
As the starting material for the core 31, a high purity SiO.sub.2
-GeO.sub.2 glass rod may be used. Its refraction index is 1.483,
while the thermal expansion coefficient is about 28.times.10.sup.-7
/.degree.C.
Following the above-mentioned surface treatment and cleaning, a
high purity glass rod with outer diameter of 10 mm and a length of
500 mm has been manufactured. This glass rod has been manufactured
into the bare fiber of the core portion with an outer diameter of
95 microns through a thermal treatment and extension at a
temperature from 1600.degree. to 1700.degree. C. in an electric
furnace.
Thereafter, it may be introduced in a single structure crucible 36
having a fine hole at the bottom and located in the high frequency
induction furnace 35 of a temperature of about 1400.degree. C.
It is apparent that although the high frequency induction furnace
35 is used in this embodiment, another heat source such as an
electric furnace, laser, gas burner, etc., could also heat the
glass material sufficiently to lower the viscosity of the material
to allow its extension.
Then, the glass cladding is formed at the surface of the bare fiber
of the core with an outer diameter of 95 microns when it passes
through the cladding glass and through the fine hole provided at
the bottom of the crucible, and thereby the core is integrated in
the optical fiber with an outer diameter of 120 microns and a core
diameter of 85 microns.
As the glass for the cladding, the multi-element glass of SiO.sub.2
--B.sub.2 O.sub.3 --Al.sub.2 O.sub.3 --Na.sub.2 O family may also
be used. The thermal expansion coefficient is 33.times.10.sup.-7
/.degree.C. which is almost equal to that of Pyrex glass, and the
refraction index is 1.472.
The composition of the glass for the cladding may also be that as
described previously above, and it is also comprehended in the
present invention to change the content and/or composition of the
additive in order to adjust the refraction index, and if required,
the thermal expansion coefficient and glass softening point.
The numerical aperture of the optical fiber is 0.16 and the optical
transmission loss is 9 dB/km for a light source with wavelength of
0.83 microns, but such transmission loss can be lowered by reducing
the scattering loss at the boundary of the core and cladding.
After formation of the cladding, silicon resin is coated on the
outside so that the outer diameter becomes 200 microns in view of
preventing generation of any fine flaws or for maintaining and
preventing deterioration of mechanical strength. At this time, the
tensile strength is about 3.5 kg/fiber.
Another embodiment of the present invention is shown in FIG. 4;
namely, the starting material 41 for the core; the device 42 for
feeding the starting material for the core; the high frequency
induction furnace 43; the ring type oxyhydrogen burner 44; the
carrier gas 45; the gas phase generation system 46; the electric
furnace 47; the glass for the cladding 48; the electric furnace 49;
the cladding glass fusing crucible 50; the polymer (primary coat)
coating device 51; the electric furnace 52; the fiber extraction
device 53; and the take-up device 54.
In this embodiment, as the starting material for the core 41, a
high purity glass rod of the SiO.sub.2 --GeO.sub.2 family is used,
and its refraction index is 1.490, while the thermal expansion
coefficient is about 32.times.10.sup.-7 /.degree.C. The starting
material 41 for the core is subjected to surface treatment and
cleaning and then manufactured into a high purity glass rod with an
outer diameter of 10 mm and length of 500 mm.
Then, it is heated and extended at a temperature of about
1650.degree. in a high frequency induction furnace and manufactured
into a bare fiber for the core with outer diameter of 110
microns.
Consequently, in accordance with the procedures of the embodiment
shown in FIG. 2, the fine granular oxide of the SiO.sub.2
--GeO.sub.2 family is deposited at the surface of the bare fiber of
the core by a fire hydrolysis method using the oxyhydrogen ring
burner 44. This oxide is thereafter heated up to about 1400.degree.
C. in the electric furnace 47 just provided in the lower side and
then sintered to the glass with formation of the cladding layer.
Thereby, a fiber with an outer diameter of 145 microns and a core
diameter of 92 microns is manufactured in an integrated core form.
The composition of the glass of the cladding layer is the SiO.sub.2
--GeO.sub.2 glass with a refraction index of 1.478 and a thermal
expansion coefficient of 25.times.10.sup.-7 /.degree.C.
Then it is coated by the glass layer through the fine hole provided
at the bottom of the cladding glass fusing crucible 50. Thus,
fibers with outer diameter of 148 microns and core diameter of 83
microns comprising the two kinds of glass cladding layers have been
manufactured.
The thickness of the glass cladding layer can be changed in
accordance with the oxide deposition onto the surface of the bare
core fiber (vapor deposition condition, burner structure and size,
etc.), but it largely depends on the viscosity of the glass and the
drawing speed, and it is possible to control the thickness of glass
cladding layer to specified dimensions.
As explained above, it is possible for the formation of the glass
cladding layer to be repeated several times using the same or
different methods, and an optical fiber having the fiber
characteristic of a graded index can be manufactured by repeatedly
applying glass layers with a different refraction index in each
layer.
In addition, this method, because the core is gradually heated and
extended each time a glass cladding layer is coated, is well suited
for manufacturing a single mode fiber having small core diameter
and thick cladding and glass layers.
The optical fiber manufactured as shown in FIG. 4 provide a
transmission loss of 7.8 dB/km at light wavelength of 0.83 microns
and numerical aperture of 0.19.
Coated fibers with outer diameter of 252 microns have been
manufactured by coating with the silicon resin. The tensile
strength of such a fiber is about 4.7 kg/fiber.
* * * * *